The traditional structural monitoring systems consist of sensors that record the state of the global and local structural stresses of the ship's structure, the pressures affecting the hull and the rigid body motions of the platform.
It is sufficient to use a limited number of sensors, the relative acquisition hardware and software conforming with the requirements of the register to obtain the additional class notation on the structural monitoring (notation MON-HULL+S of the RINA, MON HULL of the BV, HM of ABS, HMON of DNV, HSS of the Lloyd's Register).
For example, as regards the notation MON-HULL of the RINA, the following is sufficient:
1 long base strain gauge positioned on the centerline for measuring the flexural strain of the hull girder;
1 vertical accelerometer placed at the bow;
1 bi-directional inclinometer and 3 accelerometers (longitudinal, transverse, vertical) positioned at the center of gravity of the ship for the measurement of the rigid body motions.
In this case, the acquisition system consists of a panel placed at midship and containing the power supply/acquisition electronics of the sensors and an acquisition PC interfaced with the ship automation (for the acquisition of signals and for sending alarms/warnings) and with any other onboard equipment, with relative screen.
Of course, a larger set of sensors than that strictly required by the regulations allows having a more detailed picture of the state of the ship platform and of the structure, but with greater overall dimensions of the system due to the number of sensors, cables and hardware installed on board.
A more complete system consists for example of the following sensors:
6 long base strain gauges, placed at ¼, ½ and ¾ of the ship length, on the keel and below the main deck;
1 vertical accelerometer placed at the bow;
1 vertical accelerometer placed at the stern;
1 bi-directional inclinometer and 3 accelerometers (longitudinal, transverse, vertical) positioned at the center of gravity of the ship for the measurement of the rigid body motions;
4 linear strain gauges placed at critical points of the ship structure;
2 triaxial strain gauges placed at critical points of the ship structure;
1 pressure probe placed on the bottom at the bow.
In this case, the acquisition system consists of three panels—located at the stern, at midship and at the bow, respectively, so as to minimize the distance between them and the strain gauge sensors and thereby maximize the accuracy of measurement-containing the power supply/acquisition electronics of the sensors and an acquisition PC interfaced with the ship automation (for the acquisition of signals and for sending alarms/warnings) and with any other onboard equipment, with relative screen.
The use of a wave radar for the measurement of the directional sea state (wave height, direction and period) and the interaction with the onboard automation to acquire the operational parameters of the ship (propulsive state, power at the axles, propeller revolutions, activation/deactivation of equipment, etc.) and to send the alarm/warning signals regarding the structure to it allow—on the one hand—exhaustively recording the operational status of the ship so as to correlate the state of stress measured by the sensors and—on the other hand—providing the officers with an immediate feedback on the possible exceeding of the warning thresholds set concerning the structural safety of the ship.
The use of a wave radar for the measurement of the directional sea state requires the use of the onboard X-band radar or, if the use thereof is not allowed, the installation of a dedicated X-band radar, a possible signal amplification unit and a power supply unit; moreover, a control PC and a unit for digitizing the signal from the radar are required.
The presence of a calculation and computation module of the fatigue cycles in the management software of the structural monitoring system allows evaluating, through appropriate off-line post-processing methodologies, the past and residual fatigue life of the structural details gauged on board.
The “active guidance” systems (to which the system described in EP 2167916 belongs) are based on calculation tables derived from the FEM/CFD models of the ship, i.e. on the response operators of the ship motions, of the global structural stresses of the ship and of the resistance to progress.
Depending on the data relating to the sea condition and to the current route/speed (acquired by the onboard equipment), expected (acquired by weather forecast systems) or hypothetical (entered manually) conditions, these systems can provide information about the current or expected maximum ship motions in a specified period of time, about the efficiency of the ship and relative subsystems (“Safe Operating Envelope” diagram), about the route economy in the short term and the overall structural integrity (bending moment and vertical shear in the sections). The best performance of these systems is when they are interfaced to a wave radar to determine the directional sea state, preferably with vision toward the bow.
The response operators of the ship are obtained from hydrodynamic/structural numerical models of the ship which consider the hull shapes, weights, inertia, sectional characteristics, added mass thereof: the results are obtained by taking into account the static contribution in still water and the dynamic wave and whipping contributions generated by slamming.
These models therefore consider the overall response of the structure in terms of rigid body motions and bending of the hull girder and evaluate the ship efficiency in the current/expected sea weather conditions based on the current/expected maximum motions, global strains or combinations thereof in the short term.
The high reliability achieved today by the hydrodynamic/structural numerical calculation models and the progress in the forecasting and simulation techniques of the more complex hydroelastic effects (slamming/whipping) allow obtaining an excellent matching between the calculated values and the actual values: experimental campaigns carried out on tank and real models have allowed validating these approaches and making them usable in the reality of the operational management of the ship.
The onboard installation of an “active guidance system requires:                calculation of the response operators of the ship motions and of the global structural stresses of the ship concerned: rigid body motions, flexural response with wave and slamming/whipping contribution, resistance to progress, etc.;        customization of the software based on the ship concerned and the desired diagrams;        interfacing with the onboard automation for the ship position, route, speed signals, wind speed and direction, etc.;        provision of PC and screens to install the software;        provision of a radar wave system for measuring the directional sea state (X-band radar, signal amplification unit, power unit, control PC and unit for the digitalization of the signal from the radar);        possible interfacing with weather forecast systems.        
The “Safe Operating Envelopes” available on board are therefore updated in real time on the basis of the current sea weather (sea/wind) and operating conditions and are interactive, allowing the officers to enter forecast or attempt operational and/or sea weather data: the new diagram corresponding, for example, to a change in the route or speed or sea weather conditions expected within the next hours or days is overlapped by the software to the diagram of the current conditions, providing a clear and immediate visual comparison of the effects of the operational changes envisaged.
The diagrams are determined on the basis of the envelope of the limit conditions that the ship or its subsystems can withstand and may include, for example:                polar diagram for the stability of the ship;        polar diagram for takeoff/landing of helicopters (and aircraft), the so-called “helicopter/ship interface”;        polar diagram for boat hauling down/recovery;        polar diagram referred to the structural integrity of the haul girder in terms of bending moment and shearing strength at the centerline or in other sections;        polar diagram referred to the route economy in the short term, based on the evaluation of the added resistance due to the sea and the wind. Some wave radar systems are able to evaluate also the current in the navigation area, the effect of which may be included in the analysis of the route economy.        
The active guidance system thus composed bases its results only on the response operators calculated from the hydrodynamic/structural models and has small overall dimensions on board (requires no sensor).